Automated detection and location of microseismicity at Mount St. Helens with a large-N geophone array

In the summer of 2014 a dense array of 904 geophones was deployed at Mount St. Helens along the road and trail system within 15-km distance of the summit crater. The array recorded continuous data for approximately 2-weeks and presents an unprecedented seismic observation of an active volcano. A reverse-time imaging method is applied to short-term-average over long-term-average time series data to automatically detect and locate microseismicity. These efforts resulted in an order of magnitude increase in earthquake detections over the normal monitoring operations of the Pacific Northwest Seismic Network. Earthquake locations resolve a narrow, ≤1-km wide, vertical lineament of seismicity which extends from the surface to 4-km depth directly beneath the summit crater. This feature is interpreted as a fracture network that acts as a conduit connecting an underlying magma chamber to the surface.